The present invention generally relates to scanning antennas and more particularly a dual reflector design suited for scanning systems, including space-based systems.
A simple antenna scanning system uses a single paraboloidal reflector with a moveable or array feed to aim the beam over a desired field of view. Such systems are inherently disadvantageous due to the high optical aberration which causes beam degradation when scanning the beam at a moderate angle off axis.
A dual reflector design overcomes this problem by the use of a subreflector in conjunction with a primary reflector. An antenna feed is positioned so that it illuminates the subreflector. The subreflector is positioned to reflect the radiation to the primary reflector. The primary reflector then reflects the incident radiation as the desired beam. Again, a moveable or array feed is used to scan the beam off axis. The subreflector surface redirects the power from the feed to the aperture so as to correct much of the optical aberration (aperture phase error) when scanning off axis.
To design a scanned-beam dual reflector system, it is necessary to reduce optical aberration to acceptable levels. Coma, which results in a diffuse image of a point source, is a particularly troublesome aberration for beams scanned off boresight wherein the source is repositioned to effect scanning. Coma causes high sidelobes toward boresight near a scanned beam.
Conventional unblocked reflector systems include a single offset paraboloid with no correction for aberrations. Another conventional design is a coma-corrected Gregorian configuration. In a Gregorian reflector system, the reflector surfaces are not conic sections. A Gregorian configuration requires the focal array to be “vertically” placed behind the aperture, which may be undesirable for deployment.
Hence, it would be desirable to provide a compact dual reflector system that is suitable for use in spacecraft environments and earth-based applications where efficiency of packaging may be an important consideration.